The therapeutic efficacy of 25 mg/kg of cefadroxil administered once daily was compared with that of 50 mg/kg/day of ampicillin administered in four equal doses in the treatment of acute uncomplicated urinary tract infections (UTIs) in children. Nineteen girls and seven boys (mean age, 5.5 years) received cefadroxil, and 18 girls and eight boys (mean age, 5.9 years) received ampicillin. The clinical and bacteriological characteristics, as well as the demographic characteristics, of both populations were well matched. Patients with structural anomalies or with a history of hypersensitivity to cephalosporins or penicillins or abnormal hepatorenal function were excluded from the trial. Only patients with at least two consecutive positive cultures of a single pathogen, obtained in clean-catch midstream urine samples (greater than or equal to 10(5) colony-forming units per ml urine) and susceptible to the respective antibiotic, were admitted to the study. Urine cultures were repeated during the ten days of treatment and ten days after the completion of treatment. All patients in the cefadroxil group were evaluated as clinically and bacteriologically cured. Three (12%) of the patients in the ampicillin group had positive cultures in the immediate post-treatment period. The differences in the cure rates of the two groups were not statistically significant. No adverse effects of either antibiotic were observed.